Allyl group, determination

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

Among the several configurations of the crucial [Nin(octadienediyl)L] complex, all of which are in equilibrium, the p3, 1 1) species 2a and the bis(p3) species 4a are predicted to be prevalent. The odonor/71-acceptor ability of the ancillary ligand is shown to predominantly determine the position of the kinetically mobile 2a 4a equilibrium. The conversion of the terminal allylic groups via allylic isomerization and/or allylic enantioface conversion are indicated to be the most facile of all the elementary processes that involve the [NiII(octadienediyl)L] complex. Consequently, the several octadienediyl-Ni11 configurations and their stereoisomers are likely to be in a dynamic pre-established equilibrium, that can be assumed to be always present. 

Second, insertions are very selective, and the nature of the bond between organic groups and nickel determines the type of molecule which can be inserted. For example, allylic groups prefer to react with acetylene rather than with carbon monoxide (example 33, Table VII) but the opposite is true for benzyl or aryl groups. 7r-Lactonyl groups do not react either with carbon monoxide or acetylene, but they do react with ketones or aldehydes (example 39, Table VII). In this way sequential reactions take place on nickel with high selectivity. 

To determine the stereochemical relationship between the allyl group and ZnBr in intermediate 81, the reaction was performed with the more simple substrate 83, and the intermediate metallated cyclopropane 85 was quenched with allyl bromide after transmet-allation to an organocopper (equation 37). 

The addition of an allyl metal to a-amino aldehydes has been used by Vara Prasad and Rich (Scheme 12)J23l After the addition step, the allyl group is oxidized to a carboxylic acid and lactonized. Then, the a-carbon of the lactone is alkylated stereoselectively. These investigators also systematically examined the addition reaction to determine its diastereo-selectivity. The highest diastereoselectivity was obtained when allyltrimethylsilane was used in the presence of tin(IV) chloride. An increase in the steric bulk of the protecting group and of the side chain also resulted in a better diastereoselection. Alternatively, Taddei and co-workers 24-26 used a 2-(halomethyl)allylsilane and the side chains were introduced by nucleophilic substitution of the halogen (Scheme 13). 

The configuration of the C, 2 chain has been determined spectroscopically (72). Bearing in mind that the two terminal 77-allyl groups in the Cl2 chain may be mutually cis or trans (71a, 71b) to each other, and also that the molecule contains a trans double bond, the six isomers (XXXVI)-(XLI) are possible. Rotation of one of the 77-allyl groups relative to the other gives a further six possibilities this is illustrated for (XLII). This type of isomerization has been discussed elsewhere in relation to the structure of bis(77-crotyl)nickel (13). 

Strategy Both substrates have allylic groups and might react either by an SnI or an Sn2 route. The reaction mechanism is determined by the leaving group, the solvent, or the nucleophile. 

An operationally simple, yet elegant example of this type of disconnection starting from bromoisoquinoline is outlined in Scheme 12 <2002JOM(657)123>. Treatment of bromoisoquinoline with excess triallylborane provided diallyl species 63. The two allyl groups end up trans to each other but a mixture of epimers at C-4 is formed. Treatment of 63 with triethylamine generates aziridine 64 in 90% yield as a 3 1 mixture of isomers. The identity of the major isomer was not determined. 

T,CT-Perturbations of the 7r-allyl group occur in the presence of electronreleasing ligands such as phosphines, arsines, and stibines (141, 142). But the process may also take place without these agents being present, simply at elevated temperatures (143). Depending upon the concentration of donor or temperature, allyl complexes in solution may convert to the 7t,ct- or cT-bonded allyl derivatives. Ease of transformation is determined both by... 

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